Objective monitors such as the activPAL characterize time when the thigh is horizontal as sedentary time. However, there are physiological differences between lying, bent-legged sitting, and straight-legged sitting. We introduce a three-monitor configuration to assess detailed sedentary postures and demonstrate its use in characterizing such positions in free-living conditions. We explored time spent in each sedentary posture between prolonged (>1 h) versus non-prolonged (<1 h) sedentary bouts. In total, 35 healthy adults (16♀, 24 ± 3 years; 24 h/day for 6.8 ± 1.0 days) wore an activPAL accelerometer on their thigh, torso, and shin. Hip and knee joint flexion angle estimates were determined during sedentary bouts using the dot-product method between the torso–thigh and thigh–shin, respectively. Compared to lying (69 ± 60 min/day) or straight-legged sitting (113 ± 100 min/day), most time was spent in bent-legged sitting (439 ± 101 min/day, p < 0.001). Most of the bent-legged sitting time was accumulated in non-prolonged bouts (328 ± 83 vs. 112 ± 63 min/day, p < 0.001). In contrast, similar time was spent in straight-legged sitting and lying between prolonged/non-prolonged bouts (both, p > 0.26). We document that a considerable amount of waking time is accumulated in lying or straight-legged sitting. This methodological approach equips researchers with a means of characterizing detailed sedentary postures in uncontrolled conditions and may help answer novel research questions on sedentariness.

Heart rate variability (HRV) is a well-established non-invasive marker of autonomic cardiac control. We test whether time spent sitting (negatively) versus lying (positively) influences vagal HRV outcomes. HRV (10-min supine electrocardiogram) and free-living postures (dual-accelerometer configuration, 7-d) were measured in 31 young healthy adults (15♀, age: 23±3). Habitual lying (66±61 mins/day), but not sitting time (558±109 mins/day), total sedentary time (623±132 mins/day), nor step counts (10752±3200 steps/day; all, p>0.090) was associated with root mean square of successive cardiac interval differences (ρ=-0.409,p=0.022) and normalized high-frequency HRV (ρ=-0.361,p=0.046). These findings document a paradoxical negative impact of waking lying time on cardioautonomic innervation.